1. Field of the Invention
The present invention relates to a method for producing light source bulbs such as vehicular headlamps and more particularly to a process for coating the outer peripheral face of the glass tube of a light source bulb with a shielding film.
2. Description of the Related Art
Light source bulbs of vehicular headlamps are generally equipped with glass tubes and some of the glass tubes such as low-beam discharge bulbs are known to have shielding films formed on the outer peripheral faces of the glass tubes.
The shielding film is formed by applying a coating to the outer peripheral face of such a glass tube. As shown in FIGS. 7A and B, the coating application has heretofore been carried out by moving a coating application jig 204 supplied with a coating Pxe2x80x2 longitudinally along the outer peripheral face of a glass tube 4 in such a state that a light source bulb 2 has been fixed to a bulb fixing jig 202.
A single coating application jig is used to apply a coating of a given width in the conventional coating application method. However, when the shielding film is complicated in configuration, a plurality of coating application jigs have to be prepared requiring the exchange one jig for another. Therefore, coating application efficiency is poor and, moreover, it is difficult to precisely apply a coating to an estimated position.
An object of the present invention is to provide a method for producing a light source bulb wherein a coating for providing a shielding film can be applied efficiently and precisely onto the outer peripheral face of a glass tube even though the shielding film is complicated in configuration.
The invention is intended to accomplish the object above by providing an improved coating application method.
In a method for producing a light source bulb having a cylindrical glass tube extending along a reference bulb axis with a predetermined shielding film formed on the outer peripheral face of the glass tube, a process for applying a coating for providing the shielding film onto the outer peripheral face of the glass tube comprises the following steps:
horizontally placing the light source bulb, vertically placing a coating discharging portion with the bottom up and moving the front edge face of the coating discharging portion close to the outer peripheral face of the glass tube; and
applying the coating thereto by moving the coating discharging portion and the light source bulb relatively to each other along the reference bulb axis and rotating the light source bulb upon the reference bulb axis while the coating is being discharged from the coating discharging portion.
The xe2x80x9creference bulb axisxe2x80x9d means a reference optical axis of a light source bulb.
The xe2x80x9clight source bulbxe2x80x9d is not limited to any specific kind of light source bulb but includes discharge bulbs and halogen bulbs, for example, as long as they have shielding films formed on the outer peripheral faces of their glass tubes. With respect to uses, though the xe2x80x9clight source bulbxe2x80x9d is intended typically for use as a light source bulb of a vehicular headlamp, it may be intended for any other use. Moreover, the configuration of the xe2x80x9cshielding filmxe2x80x9d is not limited to any specific one.
The xe2x80x9ccoatingxe2x80x9d is also not limited to any specific kind of coating but includes coatings whose composition, color, viscosity and so on vary as long as they function as shielding films formed by coating application.
xe2x80x9cHorizontally placing the light source bulbxe2x80x9d means that the light source bulb is disposed in such a posture that its reference bulb axis extends horizontally.
As set forth above, in order to apply the coating for providing the shielding film onto the outer peripheral face of the glass tube in the method of producing the light source bulb according to the invention, the light source bulb is horizontally placed and the front edge face of a coating discharging portion vertically placed with the bottom up is moved close to the outer peripheral face of the glass tube. Further, the coating discharging portion and the light source bulb are moved relative to each other along the reference bulb axis, and the light source bulb is rotated upon the reference bulb axis while the coating is being discharged from the coating discharging portion. Consequently, the following advantage is achievable.
As the behavior of the light source bulb at the time of the coating application is represented by the rotation of the light source bulb about the reference bulb axis or a combination of rotation of the light source bulb and the movement thereof along the reference bulb axis, the light source bulb is kept horizontal at all times. On the other hand, as the behavior of the coating discharging portion at the time of coating application is equivalent to standing still or its movement along the reference bulb axis, the coating discharging portion is kept vertical with the bottom up at all times. Accordingly, the coating can be held between the front edge face of the coating discharging portion and the outer peripheral face of the glass tube in a well-balanced condition after the front edge face of the coating discharging portion is moved close to the upper end portion of the outer peripheral face of the glass tube. Thus, the coating can be applied precisely to the outer peripheral face of the glass tube by moving the coating discharging portion and the light source bulb relatively to each other in that condition.
Even in a case where the shielding film is complicated in configuration, the shielding film can be formed by applying the coating once while properly combining the movement of the coating discharging portion and the light source bulb relative to each other along the reference bulb axis with the rotation of the light source bulb about the reference bulb axis.
Although setting the diameter of the front end hole of the coating discharging portion at a small value results in increasing the distance of the movement of the coating discharging portion and the light source bulb relative to each other because the width of the coating applied becomes narrow, precise coating application can be carried out even though the shielding film is complicated in configuration. When the shielding film is not as complicated in configuration, on the other hand, the diameter of the front end hole of the coating discharging portion is set at a large value to increase the width of the coating. Accordingly, the distance of the movement of the coating discharging portion and the light source bulb relative to each other can be shortened, whereby coating application efficiency is increased.
Even though the shielding film may be complicated in configuration, the coating for providing the shielding film can thus be applied precisely to the outer peripheral face of the glass tube, according to the invention.
In order to apply the coating precisely, the width of the coating is required to be maintained as uniformly as possible and in this sense the gap between the front edge face of the coating discharging portion and the outer peripheral face of the glass tube is also required to be kept substantially constant as all times. On the other hand, the glass tube, as an object to be coated with the shielding film, is a cylindrical glass tube extending along the reference bulb axis and there are a few cases where the center axis of the cylinder does not coincide with the reference bulb axis. Even in a case where the center axis of the cylinder does not coincide with the reference bulb axis, it is common for the center axes of the cylinders of actually manufactured glass tubes to be slightly deviated from the respective reference bulb axes.
Therefore, the coating discharging portion or the light source bulb is preferably slightly displaced in the vertical direction so that the gap between the front edge face of the coating discharging portion and the outer peripheral face of the glass tube may be kept substantially constant. Even in a case where the slight displacement control is to be performed like this, the coating discharged from the coating discharging portion can be held in a well-balanced condition between the front edge face of the coating discharging portion and the outer peripheral face of the glass tube.
In order to effect the slight displacement control accurately, importance is attached to securing accurate knowledge as to how the vertical position of a region opposite to the coating discharging portion on the outer peripheral face of the shroud tube varies as the coating discharging portion and the light source bulb move relatively to each other.
Therefore, measuring an eccentricity degree of the circular section of the glass tube with respect to the reference bulb axis at two or more spots along the reference bulb axis and computing three-dimensional position data on the estimated position where the coating is applied onto the outer peripheral face of the glass tube according to the measured data make it possible to secure accurate knowledge as to how the vertical position of the region opposite to the coating discharging portion varies as the coating discharging portion and the light source bulb move relatively to each other. In other words, the slight displacement control can be effected accurately.
With the arrangement above, a coating having a viscosity of 0.1-2 Paxc2x7s (Pascalxc2x7sec) is preferred, though the viscosity of the coating is not limited to any specific value as stated above. The reason for this is that the coating applied to the outer peripheral face of the glass tube tends to sag at a viscosity of less than 0.1 Paxc2x7s, whereas when the viscosity exceeds 2 Paxc2x7s, the coating dischargeability from the coating discharging portion tends to become poor, which results in reducing the coating application efficiency.
The coating for providing the shielding film is usually composed of coating liquid containing a filler, and the coating discharging portion tends to become clogged with the coating because the filler quickly settles out; consequently, the coating liquid needs stirring to prevent this impediment. However, the coating discharging portion will become complicated in structure and also costly in a case where the coating discharging portion is provided with a stirring mechanism.
Therefore, the coating discharging portion is formed with a syringe and a predetermined amount of coating stirred in a vessel other than the syringe is sucked by and discharged from the syringe, whereby the coating discharging portion is prevented from being clogged with the coating without complicating the structure of the coating discharging portion and making any costly arrangement. The xe2x80x9cpredetermined amountxe2x80x9d means an amount to be appropriated for applying the coating once or a suitable amount exceeding the amount necessary therefor.